In applications such as TVRO ("Television Reception Only"), and DATARO ("Data Reception Only"), the signal received at the antenna of the ground station lies in a band such as 10.95-11.7 GHz or 12.25-12.75 GHz or 11.7-12.5 GHz, etc. A first stage transposes these bands to the following RF (radio frequency) bands respectively: 950-1700 MHz, 950-1450 MHz, and 950-1750 MHz.
A second transposition moves these frequencies to lie about an intermediate frequency, or IF, at a few hundred MHz, for example 450 MHz.
Each channel in the RF band is a few tens of MHz wide. The tuneable filter serves to transmit only the selected channel into the total inlet RF bandwidth. As a result, the frequencies from the local oscillator (LO) of the last transposition stage together with the image frequencies and the mixing products are rejected much more effectively.
European Patent No. EP-0138438 describes a tuneable microwave filter comprising two branches of the same type, with each end of each branch being connected to ground and with each branch comprising a series connection of a fixed inductance, a first adjustable inductance, a first variable capacitance diode, a high frequency filter constituted by a series resonant circuit comprising a second variable capacitance diode and a second variable inductance, and a coupling element comprising a series inductance between the input point and the output point of the filter.
Because of the adjustable inductances, such a filter is not cheap.
The invention seeks to mitigate this drawback.